1. Field of the Invention
The present invention relates to the polymerization of chloroprene, to the polychloroprenes obtained thereby and to the use thereof, particularly in adhesive compositions. More especially, the invention relates to the polymerization of chloroprene in aqueous emulsion, in the presence of certain chain transfer agents.
2. Description of the Prior Art
The polychloroprenes are today widely used in adhesive compositions, both in the latex state and in solution in a solvent. For such use, polymers of "sol" type are employed, namely, polymers which are soluble in organic solvents, whose viscosities may differ. However, in order to obtain adhesives which are characterized by high strengths of cohesion, high viscosities are preferred.
In most cases, for use in adhesive applications it is necessary to subject the polymer to mastication before it is placed in solution, the purpose of the mastication being to breakup the cross-linked or branched long chains which are present in the polymer and which are responsible for the elastic behavior of the product. This operation is relatively expensive. It was therefore of interest to develop a method of polymerization which makes it possible to directly obtain a rubber having the rheological properties of a standard kneaded product.
Another important standard of quality for a polychloroprene is its resistance to aging. In general, the viscosity thereof has a tendency to increase upon storage. This phenomenon, which is due to a cross-linking which is either faster or slower depending upon operating conditions, is known as aging of the polymer. The increase in viscosity may or may not be accompanied by the appearance of a gel, which results in a partial insolubilizing of the polychloroprene in preparations for adhesives and a substantial modification of the physical and rheological properties of the polymers.
It too is known that the resin acids which may be present in the polymerization system protect the latex from aging. However, these resin acids are responsible for the phenomenon of phase-separation which is observed, within a shorter or longer period of time, in adhesive mixtures having solvents comprising magnesia and phenolic resins. One way of avoiding this phenomenon consists of polymerizing the chloroprene in the presence of reduced amounts of rosin and of a moderate amount of fatty acids (see French Patent No. 2,164,095). However, under these conditions, particularly when thiurams are added upon completion of the polymerization, increased natural aging of the polychloroprene is noted.
A conventional technique which makes it possible to obtain polychloroprenes of sol type consists of polymerizing chloroprene in the presence of a chain transfer agent. The viscosity obtained depends upon the type and concentration of the chain transfer agent, as well as upon the degree of conversion of the monomer into polymer For economic reasons it is desirable that the conversion rate be as high as possible, preferably greater than 70%.
By using only a minor amount of chain transfer agent such as n-dodecylmercaptan (n-DDM), introduced in its entirety at the beginning of the polymerization, it is possible to obtain polychloroprenes of high viscosity. However, the polymer which results therefrom is of a partially cross-linked structure, which results in a "gel" phase which renders the polymer partially insoluble in its own solvents The gel appears faster the higher the final conversion.
One method which is well known in emulsion polymerization for reducing the average molecular weight of a polymer for a given conversion consists of adding the modifying agent incrementally or continuously throughout the polymerization. An improvement in this general technique has been proposed in German Patent No. 2,060,659. However, it is not possible by this technique to avoid the total absence of gel in polychloroprene latex of high solution viscosity, particularly after aging of the latex and when the conversion rate is greater than about 70%.
By "high viscosity" as utilized herein, there are intended Brookfield viscosities at least equal to about 2500 mPa.s, measured at 23.degree. C. in a 10% solution in toluene.